1. Field of the Invention
The present invention relates to an azimuth meter using spin-valve giant magneto-resistive elements, and more particularly to an azimuth meter which measures the azimuth while applying bias magnetic fields to spin-valve giant magneto-resistive elements.
2. Description of the Related Art
The present applicant already applied for a patent on an azimuth meter combining anisotropic magneto-resistive elements with a plane coil, and the application was registered as U.S. Pat. No. 6,556,007. The azimuth meter disclosed in U.S. Pat. No. 6,556,007 has a substantially square plane coil 1 and eight magneto-resistive elements 61, 62, 71, 72, 81, 82, 91 and 92 on a plane parallel and close to the plane coil as shown in the exploded perspective view of FIG. 14. Two of the magneto-resistive elements (e.g. 61 and 72) cross one side (e.g. 11) of the plane coil and the longitudinal direction of those magneto-resistive elements at substantially 45°, and the longitudinal direction of those two magneto-resistive elements are at a right angle to each other. Two magneto-resistive elements (e.g. 61 and 62 or 71 and 72) each of which crosses each of two opposed sides (e.g. 11 and 12) of the plane coil and which longitudinal directions form a right angle with each other constitute magneto-resistive element pairs 6, 7, 8 and 9. One end of each element of paired magneto-resistive elements (e.g. 6) is connected to that of the other element, and a voltage Vcc for measurement is applied between the other ends of the paired magneto-resistive elements (e.g. 6). While a biasing magnetic field is being applied by the plane coil to the magneto-resistive element pair, intermediate potential outputs of the magneto-resistive element pair are taken out from their connected ends of the magneto-resistive element pairs 6, 7, 8 and 9. An intermediate potential output difference Vx is figured out between the two magneto-resistive element pairs 6 and 7 crossing the two opposite sides 11 and 12 of the plane coil 1. An intermediate potential output difference Vy between the two other magneto-resistive element pairs 8 and 9 crossing the two other opposite sides 13 and 14 of the plane coil 1 is also figured out in a similar way. Next, while biasing magnetic fields in the reverse direction are being applied by the plane coil 1 to the magneto-resistive element pairs 6, 7, 8 and 9, similarly an intermediate potential output difference Vx of the two pairs 6 and 7 of the magneto-resistive elements crossing the two opposite sides 11 and 12 of the plane coil 1 is figured out. An intermediate potential output difference Vy between the two other magneto-resistive element pairs 8 and 9 crossing the two other opposite sides 13 and 14 of the plane coil 1 is figured out in a similar way. Then, with respect to the two magneto-resistive element pairs 6 and 7 crossing one pair of opposite sides 11 and 12 of the plane coil, the differential voltage is figured out between the intermediate potential output difference Vx earlier obtained while a biasing magnetic field was being applied and the other intermediate potential output difference Vx obtained while a biasing magnetic field was being applied in the reverse direction. Also with respect to the two magneto-resistive element pairs 8 and 9 crossing the two other opposite sides 13 and 14 of the plane coil, the differential voltage is figured out between the intermediate potential output difference Vy obtained while a biasing magnetic field was being applied and the other intermediate potential output difference Vy obtained while a biasing magnetic field was being applied in the reverse direction. The direction of an external magnetic field (e.g. earth magnetism) can be figured out from the ratio between these two differential voltages.
The resistance of a magneto-resistive element to an electric current varies with the intensity of a magnetic field applied in a direction at a right angle to the flowing direction of the current, and decreases with an increase in the intensity of the magnetic field at right angle to the current. For use in an azimuth meter, it is desirable for individual magneto-resistive elements to be substantially equal in the rate of resistance variation, and for this reason crystallomagnetic anisotropy is provided in the longitudinal direction to each magneto-resistive element of the azimuth meter described above.
In the azimuth meter disclosed in U.S. Pat. No. 6,556,007, as shown in FIG. 14, the respective longitudinal directions of the four magneto-resistive elements 61, 71, 81 and 91 and those of the other four magneto-resistive elements 62, 72, 82 and 92 are perpendicular to each other. In order to provide crystallomagnetic anisotropy to these eight magneto-resistive elements in their respective longitudinal direction, it is necessary to form the four magneto-resistive elements 61, 71, 81 and 91 in the same direction while applying magnetic fields to their longitudinal direction and, after that, to form the other four magneto-resistive elements 62, 72, 82 and 92 perpendicular thereto while applying magnetic fields to their longitudinal direction.
Moreover, since external magnetic fields (earth magnetism and the like) working in the widthwise direction of the magneto-resistive elements have to be detected, if the dimension in the widthwise direction is reduced, the influence of the demagnetizing field will increase to reduce sensitivity in measuring weak magnetic fields, such as earth magnetism. This necessitates relatively wide magneto-resistive elements, but wider magneto-resistive elements are less resistant, inviting greater power consumption in applying a voltage for measurement to the magneto-resistive element pairs. For this reason, the magneto-resistive elements should be elongated to increase their resistances, but wider and longer magneto-resistive elements would be correspondingly greater square measure, making it necessary to increase the size of the azimuth meter.